I have been wanting to try this for some time and this past week I finally gave it a go. I read that signal mirrors were one of the most underrated pieces of survival gear around so being curious I setup this test with my daughter Anne. On a clear day I can see Shafer Butte (the tallest peak on the Boise Front) from my back patio. It's 14.4 miles (as the crow flies) from my house and just shy of 5000' higher in elevation. I got out my binoculars and we sighted in on features near the summit from my home. Not wanting to introduce any high tech gear to my test, we used the old military practice of synchronizing to our timepieces. At 2:00 PM I was going to start sending a signal for 5 minutes. We allowed 5 minute window so our times were actually 1:58 PM - 2:03 PM. I arrived at my spot a few minutes early and at 1:58 PM I started signaling. At 2:05 PM I quit signaling and continued skiing for the afternoon. (As a side note, the corn snow was like silk). As soon as I got home I quickly went in the house and asked Anne if she saw my signal. The answer - YES! She first saw my signal at 1:58 PM and described it as looking like a flash bulb from a camera. Just to double check I pulled up a telephoto of the face and she pointed exactly to where I was standing.
I was quite impressed by the range of of such a small piece of glass and as a bonus they work great as a regular camp mirror. I opted to buy and use the slightly heavier glass Coghlan mirror over the lighter polycarbonate type like the SOL because of my personal bias from working with optical glass and lenses. The polycarbonate mirrors maybe equally reflective as glass (lighter and unbreakable) but I wasn't trying to test this. The big advantage of a using a signal mirror over a standard mirror is the ability to actually aim it through the targeting feature. Granted, it's takes sunlight and a fairly clear sky for a mirror to be an effective rescue tool but you never have to worry about the batteries running out of juice.